Fixes BatchedStore interval calculation

There was a bug in refactoring and interval calculation in BatchedStore. This fixes it and simplifies the interval calculation. Also fixes the Scalding unit tests to use batch cover in input space instead of store space. Fixes #567
I guess the `Interval[Timestamp]` is the maximum range covered by the `FlowToPipe[T]` while the `Interval[BatchID]` is a strict subset of that time range. So we can provide more than the Batched range. Is that right?